Originally Posted by
wizard
Fair warning you will likely get lots of conflicting answers here most of them correct for one situation or another.
You don't want to do thins unless the extrusions are specifically designed for profile rail mounting. Of course there are lots of variables here but with common framing extrusions i would consider alternatives.
If you are trying to do a precision machine build, using profile rails, trying to mount the rails directly on extrusions, is an avenue to heart ache. This is more so on a small machine where there is less spread between components thus an in ability to compensate for mis alignment.
For supported round rails you can drill your own holes in the support extrusion. This would only be useful if the flange is wide enough to line up with two slots in the T-slotted extrusion.
You might get lucky but this can easily be verified by breaking out a scale and measuring the center to center distances on the rail supports. It probably surprise nobody but these days many parts are fabricated to metric dimensions. So it is possible but maybe on extrusions to light for your needs.
The smaller the profile rails the more difficult they are to mount on T slots. Some T slots are not flat at all across the width leading to serious issues with precise alignment. You are far better off mounting the rails on a piece of tooling platen, steel bar or other interposer to support the rail.
This is generally what you want to do but it highlights a point i try to make from time to time. That is Aluminum T-slots really leave a lot to be desired for building machine tool axises on. In this case you mention that you have a much smaller machine that you are thinking about here. Now i don't know how small nor what tasks it will be doing but it might be reasonable to make the axises out of bar stock and tooling plate where needed. That bar stock could be steel or aluminum if you really wanted to use aluminum. If significantly large you probably should consider steel tubing. It is a question of what you mean by smaller, guys have built machines with work areas as small as six inches square.
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You really want to have mounting screws on both sides of the rail supports. As for mounting profile rails directly on extrusions id skip that idea unless you have extrusions specifically designed for such use
The vast majority of extrusions out there can't hold screws in the webs for crap. This is a seriously bad idea as taped holes in the webs strip out amazingly fast.
That is fine. Even with every thing said above extrusions are still useful for machine tool builds. For example if you use bar stock for the X axis rails the extrusions could be used to separate the two bars. For the gantry you might run two extrusions length wise faced with a bit of tooling plate or a wide bar. As above what makes sense depends upon what you mean by small and what you expect out of the machine.
I understand the need to use what you already have but sometimes life is easier if you settle for 75% instead of 100% usage. I know i often sound negative with respect to T slotted extrusion usage but that only reflects two concerns. One is that the stuff is expensive and two it is less than ideal for linear rail mounting. On the other hand it can be very useful to a builder that wants a kit and doesn't have a wealth of shop equipment. in the end it is up to you, as you have seen mounting the rails on T slots isn't always as easy as it might seem.